Breathing and Exchange of Gases

Every cell needs oxygen to release energy from food, and produces carbon dioxide as a waste. The process of taking in oxygen-rich air and giving out carbon-dioxide-rich air is breathing (ventilation), while the actual release of energy inside the cells is cellular respiration.

Air travels through the human respiratory system in this order: nostrils → nasal cavity → pharynx → larynx (voice box) → trachea (windpipe) → bronchi → bronchioles → alveoli. The two lungs sit in the chest (thoracic cavity), protected by the ribs and separated from the abdomen by a dome-shaped muscle, the diaphragm. The trachea is kept open by C-shaped rings of cartilage. The actual exchange of gases takes place in millions of tiny balloon-like air sacs called alveoli, which have very thin walls and a rich network of blood capillaries.

Breathing has two steps:

• Inspiration (breathing in): the diaphragm flattens (moves down) and the rib muscles raise the ribs, so the chest cavity enlarges. The pressure inside the lungs falls below the outside air, and air rushes in.
• Expiration (breathing out): the diaphragm and rib muscles relax, the chest cavity shrinks, pressure inside rises, and air is pushed out.

So breathing works by changing the volume and pressure of the chest. The rate and depth of breathing are controlled automatically by the respiratory centre in the brain.

Once fresh air reaches the alveoli, gases move between the air and the blood by simple diffusion — each gas moves from where it is in higher concentration (pressure) to where it is lower. In the alveoli, oxygen is high and carbon dioxide is low; in the blood arriving from the body it is the opposite. So:

• Oxygen diffuses from the alveoli into the blood.
• Carbon dioxide diffuses from the blood into the alveoli, to be breathed out.

The blood then carries these gases around the body.

Transport of oxygen: most oxygen (about 97%) is carried by the red pigment haemoglobin in the red blood cells, forming oxyhaemoglobin. Each haemoglobin picks up oxygen in the lungs (where oxygen is high) and releases it in the tissues (where oxygen is low). A small amount of oxygen is also dissolved in the plasma.

Transport of carbon dioxide: CO₂ is carried in three ways — mostly as bicarbonate ions dissolved in the plasma (about 70%), some bound to haemoglobin (as carbaminohaemoglobin), and a little dissolved directly in the plasma. In the lungs, the CO₂ is released from the blood into the alveoli and exhaled. This neat two-way exchange keeps supplying cells with oxygen and clearing away their carbon dioxide.

The respiratory system can be affected by infections, pollution and tobacco smoke. Knowing the common disorders helps us protect our lungs. Some important respiratory disorders:

• Asthma — the bronchi and bronchioles narrow and swell, causing difficulty in breathing, wheezing and coughing. It is often triggered by allergies, dust or cold air.
• Bronchitis — inflammation of the bronchi, with a persistent cough and mucus, often linked to smoking or pollution.
• Emphysema — the walls of the alveoli are damaged and break down, reducing the surface area for gas exchange; cigarette smoking is a major cause.
• Pneumonia — an infection in which the alveoli fill with fluid, making breathing hard.
• Occupational lung diseases — caused by breathing harmful dust at work, such as silicosis (stone/sand dust) and asbestosis (asbestos fibres).

Smoking is the single biggest avoidable cause of lung damage and lung cancer; the smoke contains many harmful chemicals, including nicotine and tar. Air pollution also damages the lungs. To keep the respiratory system healthy: avoid smoking and smoky places, reduce exposure to dust and pollution (wear a mask where needed), keep indoor air clean, and exercise to strengthen the breathing muscles. Vaccination and prompt treatment of infections also protect the lungs.